Magnetoresistive sensing device, system and method for determining a density of magnetic particles in fluid

ABSTRACT

The present invention relates to a magnetoresistive sensing device, a system and a method for determining a density of magnetic particles in a fluid. The magnetoresistive sensing device has a substrate ( 1 ) with a layer structure ( 2 ) for supporting a fluid ( 3 ). The layer structure has a first surface area ( 4 ) in a first level and a second surface area ( 5 ) in another second level and a magnetoresistive element ( 6 ) for detecting the magnetic field of at least one magnetic particle ( 7 ) in the fluid, the magnetoresistive element being positioned near a transition ( 8 ) between the first and second surface area and facing at least one of the surface areas. A corresponding system and method is described as well.

The invention relates to a magnetoresistive sensing device.

The invention also relates to a system for determining a density ofmagnetic particles in a fluid comprising the magnetoresistive sensingdevice.

The invention further relates to a method for determining a density ofmagnetic particles in a fluid using the magnetoresistive sensing device.

Chemla et al. describe in the article “Ultrasensitive magnetic biosensorfor homogeneous immunoassay”, PNAS, Dec. 19, 2000, vol. 97, no. 26 aSQUID based sensor of supermagnetic particles. The SQUID detects themagnetic flux due to magnetic nanoparticles that are present in zone ona substrate, in a well in the surface. A Mylar® sheet to which theparticles are attached so that they are immobilized, which fits in thewell, is described as an example thereof. An in-plane magnetic field isapplied to induce magnetic moments of the magnetic nanoparticles. Thenthe field is switched off. The subsequent relaxation of the magneticdipoles of the attached nanoparticles according to the Néel mechanismproduces a measurable time dependent field perpendicular to the plane ofthe immobilised zone for a period of several seconds. This field isdetected by a SQUID probe placed close to the immobilised zone.

Nanoparticles in the bulk liquid are free to rotate according toBrownian motion. In the systems studied by Chemla et al. the relaxationof the magnetic field due to this rotation is much faster than that ofthe immobilized particles. Therefore, the overall magnetic flux in theSQUID sensor produced by these non-immobilized particles in the fluid isalmost immediately reduced to zero.

A drawback of the known device resides in that the SQUID operates onlyat cryogenic temperatures. The accurate positioning of the SQUID withrespect to the immobilized zone is difficult, and the choise ofsubstrates is limited due to their small required thickness.

A major disadvantage of the known device is that it is not possible todetermine the volume concentration of magnetic particles in the fluiddue to the fast relaxation of the magnetic nanoparticles in the bulkfluid after the magnetic field has been turned off. There is a generalneed for a very accurate determination of the density of biologicalmolecules or molecular fragments, which are further called the “target”.It is therefore necessary that the density of magnetic particlesfunctioning as magnetic labels to a target can be determined with veryhigh accuracy. It would be of general interest to have an accuratesystem and method for the determination of the volume density ofmagnetic particles in the fluid.

The introduction of micro-arrays or biochips is revolutionising theanalysis of DNA (desoxyribonucleic acid), RNA (ribonucleic acid) andproteins. Applications are e.g. human genotyping (e.g. in hospitals orby individual doctors or nurses), bacteriological screening, biologicaland pharmacological research. Nowadays, there are many types of assaysused to analyze small amounts of biological molecules or molecularfragments, such as a binding assay, competitive assay, displacementassay, sandwich assay or diffusion assay. The challenge in biochemicaltesting lies in the low concentration of target molecules (e.g.fmol.1⁻¹) that has to be determined in a fluid sample with a highconcentration of varying background material (e.g. mmol.1⁻¹). Thetargets can be peptides, hormones biomarks like myoglobine, proteins,nucleic acids, steroids like cholesterol, enzymes, antigens, haptens ordrugs. The background material or matrix can be urine, blood, or serum.Other important tests are cell counting, biological coaggulation andbiological activity.

Labels improve the detection limit of a target. Examples of labels areoptical labels, colored beads, fluorescent chemical groups, enzymes,optical barcoding or magnetic labels.

It is therefore necessary that the volume density of magnetic particlesin a fluid functioning as magnetic labels to a target can be determinedwith very high accuracy.

It is an object of the present invention to provide a system of the typementioned in the openings paragraph, which is able to determine thevolume density of magnetic particles in the fluid.

The object of the invention regarding the system in accordance with theinvention is achieved in that the magnetoresistive sensing devicecomprises a substrate with a layer structure for supporting a fluid, thelayer structure having a first surface area in a first level and asecond surface area in another second level and a magnetoresistiveelement for detecting the magnetic field of at least one magneticparticle in the fluid, the magnetoresistive element being positionednear a transition between the first and second surface area and facingat least one of the surface areas. The fluid comprises a liquid or agas.

The magnetoresistive sensing device detects the net magnetic moments ofmagnetic particles. Magnetic particles being in the fluid and on thelayer structure have a magnetic moment m. The magnetic moments arealigned with a magnetic field, applied perpendicular to themagnetoresistive sensing device. When the substrate would have beenplanar, the net magnetic field in the plane of the magnetoresistiveelement in the substrate, due to randomly dispersed nanoparticles in thebulk of the fluid, would average out to zero. However, due to thetransition, there is a magnetic fringing field. The planes of the firstand second surface areas are not necessarily parallel to each other.They can make an angle with each other. The surfaces are not necessarilyflat. The transition may have a gradient profile. Transitions next toeach other may form a wave-like surface, whereby the first surface areaand second surface area can be extremely small.

In general the in plane magnetic field in the magnetoresistive elementcan be derived by integration. Just for illustrative purposes theexpression for determining the volume density is given for a transitionbeing a step from the first to the second surface areas. Due to thestepped structure of the surface, the distance d₁ from themagnetoresistive sensing device to the first surface area is differentfrom the distance d₂ to the second surface area. There is a net magneticfield in the plane of the magnetoresistive sensing element, and justunderneath the edge between the surfaces at the first and second level${H_{x} \approx {- {\frac{m}{2\pi}\left\lbrack {N\quad{\ln\left( \frac{\mathbb{d}_{2}}{\mathbb{d}_{1}} \right)}} \right\rbrack}}},$wherein N is the volume density is of magnetic particles in the fluid, mis the magnetic dipole moment per particle, and wherein x is thein-plane direction that is perpendicular to the step edge. The positivex-direction is the direction from the area with distance d₁ to the areawith distance d₂.

The magnetoresistive sensing device transforms the magnetic field in aresistance value. Because the resistance versus magnetic field of themagnetoresistance device is well known, the volume density can becalculated from the resistance value.

For calibration purposes, the resistance versus magnetic field in afluid without magnetic particles should be measured. Themagnetoresistive sensing device can be based on e.g. the GMR, TMR or AMReffect. The magnetoresistive sensing device comprises a layer structureof thin films, preferably with a linear resistance versus magnetic fieldcurve, especially for small magnetic fields, and with a negligiblehysteresis.

Preferably there is an overlap between the magnetoresistive sensingelement and the first surface area in the case of a substantiallyperpendicular projection of the first and second surface area surface onthe magnetoresistive sensing device.

The net magnetic field generated by the magnetic particles at thetransition between the first and the second surface area is strongest.Especially in the case of a sharp transition between the layers at thefirst and second level, the in-plane magnetic field in themagnetoresistive sensing device is in a first order approximation$\begin{matrix}{{{H_{x}(x)} \approx {{- \frac{mN}{2\pi}}{\int_{d_{1}}^{d_{2}}{\mathbb{d}{z\left\lbrack \frac{z}{x^{2} + z^{2}} \right\rbrack}}}}},} & (2)\end{matrix}$wherein the x is the distance defined with respect to the center of thetransition, along the direction perpendicular to the step edge directionand parallel to the first surface. The magnetic field that has to bedetected is strongest when the magnetoresistive sensing device ispresent close to transition and has a short distance to the firstsurface.

Preferably, the transition between the first and the second surface areahas a step-like profile. The equation (2) is a first order approximationfor this situation.

Generally there is also an areal density of magnetic particles presenton the structured surface. The a real density of the magneticnanoparticles on the first surface is given by σ₁, and the areal densityon the second surface is given by σ₂. The volume density of magneticnanoparticles in the fluid containing the with nanoparticles labeledelements is given by N. For a particle magnetisation along the positivez-direction, the in-plane magnetic field below the surface step can tofirst order be approximated by: $\begin{matrix}{H_{x} \approx {- {\frac{m}{2\pi}\left\lbrack {\frac{\sigma_{1}}{d_{1}} - \frac{\sigma_{2}}{d_{2}} + {N\quad{\ln\left( \frac{\mathbb{d}_{2}\quad}{\mathbb{d}_{1}} \right)}}} \right\rbrack}}} & (3)\end{matrix}$with m the magnetic moment of the particle. The areal particle densitytypically ranges between zero and 10³ to 10⁴ particles per μm².

The volume density can be calculated from a combination of the unequaloutput signals of several unequal magentoresistive sensing devices, asexplained below.

If the areal density on the first surface is unequal to the secondsurface, the areal densities and volume density can be determined fromthe structured surface, having at least three structures with an unequalcombination of depths d₁ and d₂. The surface structures each have anidentical magnetoresistive sensing device, being present in the sameplane, at the same position relative to the each step. Due to thedifferent heights of the steps, the three different output signals fromthe sensing devices make it possible to derive the surface areal densityand the concentration of magnetic particles. In the special case inwhich the areal densities at the first and second surfaces are equal theareal and volume density can be determined from a structured surface,having at least two structures with an unequal combination of depths d₁and d₂.

In a binding assay, a competition assay, or a displacement assay,generally there is a probe area present on the first surface area.Attached to the probe area are binding sites.

In a binding assay, the magnetic particles are coupled to a targetforming magnetic labels. The magnetic labeled target diffuses throughthe fluid by Brownian motion. Small particles diffuse faster and reachthe binding sites faster than large particles. The magnetic labeledtarget is bound at the binding site.

In a competition assay, there are targets of which the concentration isto be determined and magnetically labeled targets present in the fluid.The two species compete for binding to capture molecules. Differences inabundance and binding kinetics (diffusion, binding efficiency) determinethe relative binding of the species to the capture molecules. The moremagnetically labeled target is present at the binding sites, the lesstarget was present in the test volume.

In a displacement assay magnetic labeled targets are bound at thebinding sites. Target in the volume diffuses to the binding sites andreplaces the magnetic labeled target molecules. The larger theconcentration of target in the fluid, the less magnetic labeled targetis detected.

As given by equation (3), a determination of the volume density ofmagnetic particles N requires that the effect of the areal density ofmagnetic particles on the surfaces at levels one and two is eliminated.

The detection accuracy can be improved when the magnetoresistive sensingdevice has a Wheatstone bridge configuration comprising magnetoresistiveelements being positioned on the substrate. The first bridge-half may belocated below a first transition between the a first surface area and asecond surface area, and the other bridge-half being located below asecond, dissimilar, transition between surface areas, such that thechanges of the resistances of the two sensors, upon the application ofthe external field, are different.

The detection accuracy can further be improved when the pairs of firstand second magnetoresistive sensing devices or groups of first andsecond magnetoresistive sensing devices are used, each pair beingassociated with and located with a transition, the outputs of the firstand second magnetoresistive devices being fed to means for detecting achange in resistance of the magnetoresistive sensing devices upon theapplication of the external field.

In an advantageous embodiment, the layer structure is formed by aplurality of grooves, positioned parallel to each other. Themagnetoresistive sensing elements are made of a material of which theresistance versus field curve is to a first approximation a linearfunction of the x-component of the applied field. The elements arepresent in the substrate and are substantially stripe-shaped andcentered along the edges of the grooves. The distance between the grooveedges is larger than the width of the stripe shaped magnetoresistiveelements.

In a second advantageous embodiment, the layer structure is also formedby a plurality of grooves, positioned parallel to each other. However,the magnetoresistive sensing elements are now made of a material ofwhich the resistance versus field curve is to a first approximation asymmetric function of the x-component of the applied field. It is onlysensitive to the absolute value of the x-component of the field, and notto its sign. The elements are present in the substrate and can have adimension in the x-direction that is much larger than the distancebetween the edges between the grooves. In that case the precise positionof the grooves with respect to the element is not of a criticalimportance. Alternatively, when the dimension of the magnetoresistivestripe-shaped elements is of the order of the distance between thegroove edges, or smaller, the stripe-shaped sensor elements are centeredalong the edges of the grooves.

This provides the ability to analyze small amounts of a large number ofdifferent molecules or molecular fragments in parallel, in a short time.One biochip can hold for 1000 or more different molecular fragments. Itis expected that the usefulness of information that can become availablefrom the use of biochips will increase rapidly during the coming decade,as a result of projects such as the Human Genome Project, and follow-upstudies on the functions of genes and proteins.

In case extremely large numbers of target species have to be analyzed,for instance from one fluid test sample, the substrates with the groovescan be stacked, forming a three dimensional array of channels. Thisallows a very compact detection system. Especially when the dataprocessing occurs in the substrates stacked on each other.

Preferably the means for detecting a change in magnetoresistance of themagnetoresistive sensing devices comprise an integrated circuit. Anelectronic integrated circuit can easily be manufactured in thesubstrate. Especially when the substrate is a semiconductor,conventional techniques can be used to obtain electronic devices likeMOSFET, bipolar transistors, diodes, optical devices or a variety ofsensors such as temperature sensors, ion sensitive electrodes, pressuresensors, viscosity sensors, flow sensors and current sensors or voltagesensors.

It is another object of the present invention to provide a system thatdoes allow for determination of the volume density in the fluid of thetest sample.

The object of the invention regarding the system in accordance with theinvention is achieved in that a magnetoresistive sensing device is usedcomprising a substrate with a layer structure for supporting a fluid,the layer structure having a first surface area in a first level and asecond surface area in another second level and a magnetoresistiveelement for detecting the magnetic field of at least one magneticparticle in the fluid, the magnetoresistive element being positionednear a transition between the first and second surface area and facingat least one of the surface areas, and an electronic circuit fordetecting a change in magnetoresistance of the magnetoresistive sensingdevices, the electronic circuit being present in the substrate.

The electronic circuit may comprise a differential comparator circuit.

It is another object of the present invention to provide a method thatdoes allow for determination of the volume density in the fluid of thetest sample.

The object of the invention regarding the method in accordance with theinvention is achieved in that a magnetoresistive sensing device is usedcomprising a substrate with a layer structure for supporting a fluid,the layer structure having a first surface area in a first level and asecond surface area in another second level and a magnetoresistiveelement for detecting the magnetic field of at least one magneticparticle in the fluid, the magnetoresistive element being positionednear a transition between the first and second surface area and facingat least one of the surface areas, the method comprising the steps of

-   -   providing a fluid comprising magnetic particles over the layer        structure    -   applying a magnetic field    -   sensing the magnetoresistive sensing element, while applying the        magnetic field    -   comparing the output signal from the magnetoresistive sensing        device to a reference signal taken at zero applied field to        thereby determining the volume density of magnetic particles.

The volume density of magnetic nanoparticles is derived from theresistance change of the magnetoresistive sensing device upon theapplication of a perpendicular magnetic field, which follows from thechange of the voltage difference over the device upon the application ofthe magnetic field when the sensor is operated at a constant sensecurrent.

In general, it is required to determine independently the particledensities on the surfaces and in the bulk. This can be achieved bycombining the measurements of several sensors integrated on the samechip or in the same device, the sensors having different surfacestructures, e.g. different values for d₁ and d₂. The data from an arrayof sensors can then be combined to yield accurate values of σ₁, σ₂ andN.

Equation (3) can be used for that purpose if the width of themagnetoresistive sensing element is much smaller than the distance d1and d2. This is generally not the case, since the width of the sensingelement can be of the same magnitude as the distance d1 and d2. Thederived equations are therefore just to illustrate the principle of theinvention.

-   -   If the areal densities are equal σ₁=σ₂, the volume density N is        proportional to the difference in resistance between a first        magnetoresistive sensing element corresponding to a first        surface structure and a second sensing element corresponding to        a second surface structure having a third and fourth surface        level, if (1/d1−1/d2)=(1/d3−1/d4) and d2/d1 is not equal to        d4/d3. The distances d3 and d4 being the distance of the second        sensing element to the third and fourth surface respectively.    -   If the areal densities are not equal: σ₁=/σ₂, a third sensing is        necessary. For example d6/d5 is equal to d2/d1, but d6, (d5) is        not equal to d1,(d2) respectively. From the difference in        resistance values between the sensing elements sensoren with        d1/d2 en d5/d6 follows (σ₁/d1−σ₂/d2), so the measured magnetic        field of the d1/d2 sensor can be corrected.

These and other features and advantages of the device according to theinvention will become apparent from the following detailed description,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention. This description isgiven for the sake of example only, without limiting the scope of theinvention.

The reference figures quoted below refer to the attached drawings.Therein is:

FIG. 1 a schematic cross sectional view of the magnetoresistive sensingdevice according to the invention.

FIG. 2 a schematic top view of the magnetoresistive sensing deviceaccording to the invention.

FIG. 3 a graph illustrating the x-component of the magnetic field of thenanoparticles in the plane of a GMR sensing element.

FIG. 4 Schematic cross-section of the magnetoresistive sensing device,having a volume density and areal densities of magnetic particles.

FIG. 5 a graph of the response of a multilayer GMR sensor element to anapplied field according to the present invention.

FIG. 6 schematically shows a multi-step structure above a plurality ofmagnetic sensor elements.

FIG. 7 a schematic cross sectional view of the system according to theinvention.

FIG. 8 schematically shows a cross sectional view of a layer structure,with surface areas in the first to sixth level.

The magnetoresistive sensing device in FIG. 1 comprises a substrate (1)with a layer structure (2) for supporting a fluid (3). The layerstructure has a first surface area (4) in a first level and a secondsurface area (5) in another second level and a magnetoresistive element(6) for detecting the magnetic field of at least one magnetic particle(7) in the fluid (3). The magnetoresistive element (6) has beenpositioned near a transition (8) between the first and second surfacearea and facing at least one of the surface areas (4,5).

The fluid comprises a target molecule species or an antigen.

Any biological molecule that can have a magnetical label can be ofpotential use in this application.

In FIG. 2 the width 10 and length 11 of the magneto-resistive (MR)sensor element is much larger than the diameter of the magneticnanoparticles of which the presence and concentration is to be measured.

The nanoparticles may for example have a diameter between 1 and 250 nm,preferably between 3 and 100 nm, most preferred between 10 and 60 nm.For such small particles, the diffusion is fast. The width and lengthdimensions of sensor elements are at least a factor 10 or more,preferably a factor 100 or more, larger than the diameter of thenanoparticles, for example 1 μm×1 μm. Other dimensions for the sensorelements are also possible. If different dimensions are used, differentS/N ratios are obtained.

The magnetoresistive sensing element 6 comprises thin film materials, inthe example under reference GMR materials, but also other thin filmmaterials such as AMR, TMR or other MR materials with substantiallylinear R(H) curves around H=0 are possible materials. The sensingelement is separated from the magnetic nanoparticles 7 by a layer e.g.silicon dioxide, silicon nitride, or an organic material such as aresist or epoxy for example.

In a first embodiment of the magnetoresistive sensing device, themagnetoresistive sensing element consists of a GMR strip. The GMR stripof the sensing element can be a meander, resulting in a larger area andimproved sensitivity. If a magnetic field is applied perpendicular tothe magnetoresistive sensing element 6, there is a fringing field at thetransition between the first and the second surface area of the layerstructure. In FIG. 3 is shown that the magnetic field in the x-directionof the sensing element is largest if the center of the sensing elementis below the center of the transition 8.

The magnetisation of the nanoparticles 7 is controlled by an externalfield applied perpendicular to the magnetoresistive element (i.e. alongthe z-axis) as shown in FIG. 4. The magnetoresistive element is nowexposed to the magnetic field resulting from the nanoparticles 7.

For a particle magnetisation along the positive z-direction, thein-plane magnetic field below a step-like transition between the firstand second surface area can to first order be approximated by:$H_{x} \approx {+ {\frac{m}{2\pi}\left\lbrack {\frac{\sigma_{1}}{d_{1}} - \frac{\sigma_{2}}{d_{2}} + {N\quad{\ln\left( \frac{\mathbb{d}_{2}\quad}{\mathbb{d}_{1}} \right)}}} \right\rbrack}}$with m the magnetic moment of the particle. The areal density of themagnetic nanoparticles 7 on the first surface area is given by densityσ₁, and on the second surface area by density σ₂. The volume density ofmagnetic nanoparticles 7 in the fluid containing the with nanoparticles7 labeled elements is given by N.

In the special case that the areal density is almost zero, the density Nof magnetic particles is derived from the resistance of the sensingelement. A typical output signal from the magnetoresistive element 6 isshown in FIG. 5. The concentration N is determined from the value of themagnetic field H_(x), the known magnetic moment of a magnetic particlewith a diameter of 35 nm being m=3.10¹⁸ Am², and the distance d1, d2 andthe formula$H_{x} \approx {+ {{\frac{m}{2\pi}\left\lbrack {N\quad{\ln\left( \frac{\mathbb{d}_{2}\quad}{\mathbb{d}_{1}} \right)}} \right\rbrack}.}}$

FIG. 6 shows a multiple step structure and many sensing elements. Over alarge area the volume density can be determined.

In FIG. 7 a structure is shown which has many levels on top of thesubstrate. This structure is useful for the determination of uniformareal density. In addition to the physical structure, the first andsecond surface areas can also be chemically structured. For instance thesurface of the layer structure is plasma polymerized. For instance PEGs(polyethylene) are provided uniformly on the surface. Capture moleculescan for example be antibodies, antibody fragments, receptors, ligands,nucleic acids or oligonucleotides. The capture molecules are chemicallyor physical-chemically provided over the surface. These capturemolecules are able to selectively bind a target.

The volume density N can be determined from the difference of the outputsignals of the magnetoresistive sensing elements 6 and 6′.

In the special case that 1/d1−1/d2=1/d3−1/d4, the difference of thesignal yields −m/2π.N(ln d2/d1−ln d4/d3). From the known distances tothe surfaces, the volume concentration N is determined. For example thedistances can be d1=1 μm, d2=2 μm, and d3=2/3 μm, d4=1 μm.

The output signals from the magnetoresistive sensing elements can beamplified, for instance with a differential amplifier. In thesemiconductor substrate 1, there are many semiconductor devices presentwhich form an electronic circuit 30, such as bipolar transistors,mosfets and diodes.

In an advantageous method, a magnetic field is applied perpendicular tothe magnetoresistive sensing elements. The magnetic field has amagnitude of typically 100-1000 Oe and is switched.

In a first step the uniformity of the magnetic field is optimized. At atime t1=0 the current through the coil or coils is registered. Themagnetoresistance value of the sensing elements are registered. At asecond time t2, the magnetoresistance curve of the sensing elements 6and 6′ is detected again. If there is a large difference between theoutput R(H) curves of the sensing elements 6 and 6′ between t1=0 and t2,this indicates that the position of the particles is changed due to theforce on the particles that results from a field gradient. The magneticfield must then be tuned, for instance with small coils which yield acompensation field gradient.

Having a uniform magnetic field, the magnetic field is on during atypical time of 1 ms. During this time the magnetoresistive sensingelements 6 and 6′ are sensed by sending a current through the devices.From the difference in output signals the volume density and arealdensity is determined. It is of large importance to have the outputsignals of the magnetoresistive sensing elements at zero magnetic fieldas a reference. The reference resistance values are determined with alock-in technique. It is also possible to measure for instance during 1ms at zero magnetic field, 1 ms at a magnetic field H and again 1 ms atzero magnetic field to obtain a reference.

When the magnetic field is off, the relaxation is very fast and occursalmost immediately. This relaxation time is much smaller than 1 ms.During the time the magnetic field is switched off, the magneticparticles diffuse very rapidly. The diffusion length is much larger thanthe distance over which the magnetic particles attract each other. In 1ms, the diffusion distance of a magnetic particle with a diameter of 35nm is about 0.15 micrometer, while the distance of attraction for avolume density of 1 nmol/l is about 10 picometer for a particle with amagnetic moment m=3.10¹⁸ Am².

During the off-time, a uniform distribution of magnetic particles isreached. The time the magnetic field is off is typical 1 ms. In severalareas of the sample can be measured as a function of time.

During a time of 3 minutes, at least 100 measurements can be done.

With this procedure, the accuracy with which the volume concentrationcan be detected is at least 1 nmol/l. The areal particle densitytypically ranges between zero and 103 to 10⁴ particles per μm².

In order to determine the volume density as well as the areal density, asecond layer structure is present as shown in FIG. 7. The second layerstructure has a third surface area in a third level and a fourth surfacearea in a fourth level.

The second layer structure corresponding a second magnetoresistiveelement being positioned near a transition between the third and fourthsurface area and facing at least the third surface area.

From equation (3) it can be derived that in case of an uniform arealdensity σ₁=σ₂, the distance from the magnetoresistive sensing elementsto the surface must be1/d 1−1/d 2=1/d 3−1/d 4.

The difference in magnetic field H_(x1)−H_(x2)=−m/2π. N (ln d2/d1−lnd4/d3).

From the difference in magnetoresistance and the known R(H)characteristic, the volume density can be extracted. From themagnetoresistance signal of the first Wheatstone bridge, the arealdensity can be determined.

From FIG. 8 and equation (3) it can be derived that in case of an arealdensity whereby σ₁≠σ₂, the distance from the magnetoresistive sensingelements to the surface must be:1/d 1=1/d 3+1/d 51/d 2=1/d 4+1/d 6 and d 1≠d 2, d 3≠d 4 and d 5≠d 6

The difference in magnetic field H_(x1)−H_(x2)−H_(x3)=m/2π. N (lnd1/d2−ln d4/d3−ln d6/d5).

From the difference in magnetoresistance and the known R(H)characteristic, the volume density can be extracted. From themagnetoresistance signal of the first and second Wheatstone bridge, theareal density σ₁, and σ₂ can be determined.

1. A magnetoresistive sensing device, comprising a substrate (1) with alayer structure (2) for supporting a fluid (3), the layer structure (2)having a first surface area (4) in a first level and a second surfacearea (5) in another second level and a magnetoresistive element (6) fordetecting the magnetic field of at least one magnetic particle (7) inthe fluid (3), the magnetoresistive element being positioned near atransition (8) between the first and second surface area and facing atleast one of the surface areas.
 2. A magnetoresistive sensing deviceaccording to claim 1, wherein there is an overlap (9) between themagnetoresistive sensing element (6) and the first surface area (4) inthe case of a substantially perpendicular projection of the first (4)and second (5) surface areas on the magnetoresistive sensing element(6).
 3. A magnetorsistive sensing device according to claim 2, whereinthe magnetoresistive sensing element (6) is centered around thetransition (8), seen in substantially perpendicular projection.
 4. Amagnetoresistive sensing device according to claim 1, wherein thetransition (8) has a step-like profile.
 5. A magnetorsistive sensingdevice according to claim 1, wherein the magnetoresistive sensing devicehas a Wheatstone bridge configuration comprising magnetoresistiveelements being positioned on the substrate (1).
 6. A magnetorsistivesensing device according to claim 1, wherein a second layer structure(11) is present, the second layer structure having a third surface area(14) in a third level and a fourth surface area (15) in a fourth level,and to the second layer structure (11) corresponding a secondmagnetoresistive element (6′) being positioned near a transition (8′)between the third and fourth surface area and facing at least the third(14) surface area.
 7. A magnetorsistive sensing device according toclaim 1, wherein the magnetoresistive element (6) is present on thesubstrate (1).
 8. A magnetorsistive sensing device according to claim 1,wherein the structured surface (4,5) is formed by a plurality of grooves(12), positioned parallel to each other.
 9. A magnetorsistive sensingdevice according to claim 8, wherein structured surfaces (4,5) arestacked on top of each other, forming a three dimensional array ofchannels.
 10. A system for determining a density of magnetic particlesin a fluid, the system comprising the magnetorsistive sensing deviceaccording to claim
 1. 11. A system according to claim 10, furthercomprising an electronic circuit (30) for detecting a change inmagnetoresistance of the magnetoresistive sensing devices, theelectronic circuit being present in the substrate.
 12. A systemaccording to claim 10, further comprising means (40) for generating amagnetic field.
 13. A method for determining the density of magneticparticles in a fluid using the device of claim 1, the method comprisingthe steps of providing a fluid (3) comprising magnetic particles (7)over the layer structure (2) applying a magnetic field sensing themagnetoresistive sensing element (6), while applying the magnetic fieldcomparing the output signal from the magnetoresistive sensing element(6) to a reference signal obtained in the absence of an applied field tothereby determining the volume density of magnetic particles (7).
 14. Amethod according to claim 13, comprising the steps of: sensing a secondmagnetoresistive element (6′) corresponding to a further second layerstructure (11) having, the further layer structure having a thirdsurface area (14) in a third level and a fourth surface area (15) in afourth level compare the output signals from the magnetoresistivesensing elements (6, 6′) corresponding to the first (2) and second (11)layer structures, and compare these signals to the signals determined inthe absence of an applied field, to thereby determine the volume densityand areal density of magnetic particles (7).
 15. A method according toclaim 14, comprising the steps of: providing a probe area (50) having abinding site on the first surface of the layer structure beforeproviding the fluid, sensing a third magnetoresistive element (6″)corresponding to a further third layer structure (18), the further thirdlayer structure having a fifth surface area (19) in the fifth level anda sixth surface area (20) in a sixth level, and comparing the outputsignals from the magnetoresistive sensing devices (6,6′,6″)corresponding to the first, second and third layer structures with andwithout the applied field, to thereby determine the volume density, theareal density of magnetic particles present on the second surface (5)and the areal density of magnetic particles present in the probe area onthe first surface (4).
 16. A method according to claim 13, wherein thefluid is blood.
 17. A method according to claim 13, wherein the fluid isurine.
 18. A method of detecting a concentration of magnetic particlescoupled to a target in a fluid, making use of the magnetoresistivesensing device as claimed in claim 1, the method comprising the stepsof: providing a fluid (3) comprising the target over the layer structure(2) applying a magnetic field sensing the magnetoresistive sensingelement (6), while applying the magnetic field comparing the outputsignal from the magnetoresistive sensing element (6) to a referencesignal obtained in the absence of an applied field to therebydetermining the volume density of magnetic particles (7).
 19. A methodas claimed in claim 18, wherein the fluid is blood.
 20. A method asclaimed in claim 18, wherein the fluid is urine.
 21. A system as claimedin claim 10, wherein the system is a blood tester.